THICKNESS MEASURING OF NICKEL COATINGS ON STEELS WITH VARIOUS MAGNETIC PROPERTIES

Transcription

1 THICKNESS MEASURING OF NICKEL COATINGS ON STEELS WITH VARIOUS MAGNETIC PROPERTIES A. A. Lukhvih, O. V. Bulatov, A. L. Lukyanov, and A. A. Polonevih Institute of Applied Physis of the National Aademy of Sienes of Belarus, Minsk, Belarus, During realization of the nondestrutive testing of thikness of nikel oatings, the main interfering fator is the strong dependene of physial properties of the oatings on the strutural state of nikel. If nikel is oated on a nonmagneti base, at the expense of use of the magnetodynami method [-], auray of the testing is onsiderably growing, resolution is inreasing, and measurement range is widening. Aording to the results reeived earlier [], the magneto-dynami transduer with optimized parameters provides testing of nikel thikness up to µm and more with the error ~ %, and the resolution at the beginning of the range is several tenth parts of mirometer. The situation beomes onsiderably ompliated when the base has magneti properties. In this ase beause of an informative signal is aused by magnetization not only of the nikel oating, but also the base, the signal ontains the non-informative omponent, value of whih onsiderably depends on properties of the base and the over. Hene, while passing from the nonmagneti base to magneti, inevitably, the resolution is dereasing and the measurement range is shortening. By virtue of it, the range that is provided by modern magneti thikness gauges while testing of nikel oatings on low-arbon steels does not exeed µm. The investigations for substantiation of a possibility of testing of nikel oatings on steels with different magneti properties in a number of ases that important for pratie have not been arried out until now. In this paper, the results of researhes of possibilities of the magneto-dynami method that is used for testing of nikel oatings on steels with properties, whih vary from pratially nonmagneti to strong-magneti are presented. In partiular, the data of harater of magnetization distribution in the oating and base at different value of the primary magnetizing field are given, as well as the data of dependene of the resolution and measurement range on size of this field are offered. For produing a number of details that are oated by layers of galvani nikel, steel grades Х8НТ, ХНТ, ХНМВФБ, and ХНМТР-ВД are used. Beause the information about magneti properties of these materials in literary soures has not been found, their normal magnetization urves were got experimentally. For this purpose from named steels and from steel in a ondition of delivery, pairs of idential diss were made. From a dis of eah pair the ring was ut out; for all rings, external diameter was more internal approximately in, times. On eah ring turns of a measuring winding by a diameter of, mm, then turns of a magnetizing winding by a diameter of, mm were uniformly applied. By this proedure, five ring speimens were made. Alexander Osipov in Vladimir Matiuk s laboratory on the ertified apparatus УИМХ- [] arried out investigations of their magneti properties. The normal magnetization urves of all speimens are presented in fig. ; there for omparison the urve of pure nikel is shown. В, T,, 8 Н, ka/m Fig.. The normal magnetization urves of materials of the oating and base: urve (it pratially interflows with the axis H) steel Х8НТ, steel ХНТ, pure nikel, steel ХНМВФБ, steel ХНМТР-ВД, and steel

2 Judging by harater of the urves in fig., the properties of the steels under onsideration vary in the extremely wide range from pratially nonmagneti (steel Х8НТ) to strongmagneti (steel ХНМТР-ВД), and the properties of steels Х8НТ and ХНТ are weaker than nikel properties, but the properties of other steels are stronger. Hene, in pratie while testing of thikness of nikel oatings on suh bases is arrying out, one should expet very big variation of the resolution and measurement range. The urves shown in fig. were used for the omputations made by means of the finite element method. All simulated magneto-dynami transduers had the ommon onstrution arrangement but differed in magnet energy (the produt of speifi energy of material of a magnet on its volume). In a role of produts under test, models of the diss having an equal diameter were used; eah dis onsisted of the base, material of whih was one of five mentioned steels, and the nikel oating by thikness up to µm. The distribution of magnetization of the oating and base with different value of the primary magnetizing field was researhed. The module of this magnetization was omputed in the oating along its all thikness and in the base on the depth µm. In fig. the distribution of the magnetization module for the transduers with magnet energy about and 8 mj when base thikness is µm and nikel thikness is µm is presented. IJI, A/m IJI, A/m 8 r, mm 8 r, mm a b r, mm r, mm z, mm z, mm Fig.. The distribution of the module IJI of magnetization along the radius r and depth z with different magnet energy: a in the oating, b in the base on depth µm, and along an axis of symmetry of the dis model; the plots at the left magnet energy is about mj, on the right about 8 mj; urve steel Х8НТ, steel ХНТ, steel ХНМВФБ, steel ХНМТР-ВД, and steel a and b

3 The harts show that strengthening of the magnetizing field always results in inreasing of magnetization of the base and oating. At a weak magnetizing field, onsiderable strengthening of magneti properties of the base results in inessential growth of its magnetization, while at a strong field, magnetization of the base an appear more than oating magnetization. It is aused by that even at enough big magnet energy, magnetization of the oating in the area that is diretly under the pole tip of the transduer annot exeed the saturation magnetization of nikel (it is about, MA/m), while magnetization of the base that is steel an reah MA/m. It is lear that further strengthening of properties of the base results in its even more magnetization. Beause the noninformative omponent ontained in the transduer signal and aused by the seondary field of the magnetized base is mainly formed by the base properties, a transduer with any magnet energy on steel provides the measurement range that is muh narrower than the range on steel Х8НТ. Said was proved by the omputations results that were obtained while the investigation of dependene of the informative signal (the flux of indution of the seondary magneti field) on thikness of nikel oatings on bases with different magneti properties when the value of a primary magnetizing field was different was being arried out. Fig. shows the dependenies represented for the transduers with magnet energy about,, and 8 mj. F, µwb F, µwb a,,8,,,, b, mm b,,, b, mm,,,,8 b, mm Fig.. The dependene of the informative signal F on the thikness b of nikel oated on bases with different magneti properties: a the model of the magnetodynami transduer with magnet energy ~ mj, b ~ mj, and ~8 mj; urve steel Х8НТ, steel ХНТ, steel ХНМВФБ, steel ХНМТР-ВД, and steel One an see that the sensitivity to nikel thikness depends on magneti properties of the base in greater degree than on value of the magnetizing field. While properties of the base are strengthening, the measurement range is promptly narrowing, and when these properties are pratially idential with properties of the oating, the testing of nikel thikness is on priniple impossible irrespetive of value of the magnetizing field. One an see also that if magneti properties of nikel are stronger than properties of the base, the funtion F = f (b) at any magnet energy is monotonially inreasing until the measured thikness omes up to depth of the informative area. Beause this depth is proportional to value of the magnetizing field, in the ase of the nonmagneti base or very lose to it, by the instrumentality of the transduer with the weakest magnet one an test nikel oatings by thikness no more than µm while with the strongest magnet by thikness approximately up to µm. If magneti properties of the over are stronger than nikel properties, the situation is hanging on priniple and the mentioned funtion from the dereasing urve quikly turns into the straight line parallel to the axis b. One an see that in this ase, the sensitivity to nikel thikness with any magnetizing field is maximal only if properties of the base are strongest (steel ). The omputations show that on suh base the

4 transduers with magnet energy about and mj provide testing of nikel by thikness up to µm while the transduer with the biggest energy by thikness no more than µm. If one takes into aount that with magnet energy no less than 8 mj omplete elimination of the main interfering fator (the strutural state of nikel) is pratially provided, the measurement range on bases with weak magneti properties is no less than µm and the testing error is minimal. If properties of the base are stronger than nikel properties, while hoosing the transduer the ompromise between width of the range and value of the error is neessary. The omputations truth was proven by the measurements made on five diss, whih remained after produing the ring speimens. During the measurements, nikel plates replaing galvani nikel layers by different thikness were being imposed over the diss. Beause the majority of the plates had been made by rolling with plasti deformation up to %, and other by milling, grinding, and polishing, the variation of strutural properties of nikel was signifiant. As a measurement instrument, the magneto-dynami transduer with magnet energy ~ mj onneted to an eletroni module of a thikness gauge was used. The results of the measurements are presented in fig.. F, dimensionless units Fig.. The dependene of the signal F of the transduer with magnet energy about mj on the thikness b of the nikel plates imposed over the steel diss with different magneti properties: urve steel Х8НТ, steel ХНТ, steel ХНМВФБ, steel ХНМТР-ВД, and steel,,,,8 b, mm Despite of learanes that were unavoidably between the plates and diss beause of nonflatness of their surfaes, as well as variation of nikel struture, the experimental urves in their harater are idential to the theoretial urves. On the one hand, it is the evidene of truth of the results that were obtained earlier; on the other hand, it indiretly affirms the possibility of partial elimination of nikel struture while using the transduer with the average magnet energy. Basing on the reeived results, one an state that the magneto-dynami method provides the testing of thikness of nikel oatings on bases with different magneti properties, with the exeption of the ase when base properties and nikel properties are pratially equal. On the one hand, the measurement range is defined by properties of the base; on the other hand, this range is defined by the magnetizing field value that is depended on parameters of the used transduer. If properties of the base are speified, the transduer reating the magnetizing field that has optimal value provides testing of nikel thikness in the maximally wide range. The results given in the paper show the possibility of development of magneto-dynami gauges for thikness testing of nikel applied on bases with different magneti properties. These results an be used for transduers parameters optimization that is neessary for arrying out measurements in the defined ranges with the required auray. REFERENCES. Useful model patent BY, IPC G R /. The Devise with Mounting Magnets for Nondestrutive Testing / A. A. Lukhvih, A. L. Lukyanov. u ; patented..; published.. // Inventions. Useful Models. Industrial Models. Offiial Journal,,, Part. P..

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